1. Field of the Invention
The present invention relates to systems and methods for remote measurement of geometric features on an object, and, more particularly, to such systems and methods that are photogrammetrically based.
2. Description of Related Art
Many industrial processes and applications require an accurate description of various large, complex surfaces. Surfaces that are measured for the purpose of comparison with a desired design, quality control, or deformation evaluation include automobile parts, airplane fuselages and wings, and antenna dishes, to name but a few.
A well-established process of surface measurement that has been in use for decades includes placing a plurality of targets on a surface in a desired pattern and determining the position of the targets by photogrammetric techniques. At least two cameras of known relative position and orientation image the targets on a two-dimensional medium, such as a film or a digital-image sensor. The three-dimensional position is determined by a process of triangulation (or intersection). Known shortcomings of this technique for many applications include a need to place, and later remove, targets on the object and a practical limit on the number of targets.
Various techniques for overcoming the targeting difficulties are known in the art, wherein physical targets are replaced by dots of light cast onto a surface to be measured by a projector. These dots originate on a glass plate or another type of projectorslide, from which they are projected by a light source through a lens system onto the surface. The projected points may form a dense grid, perhaps into the thousands.
If two cameras image simultaneously with the flash of the projector, in theory they can identify and triangulate each projected point. The identification is often accomplished by a technique called epipolar matching, which seeks intersecting or nearly intersecting rays. When the number of projected points is great, the matching becomes ambiguous owing to the possibility of many rays from both cameras lying nearly in the same plane and thus offering a number of potential matches.
Resolution of most of the inherent ambiguities may be accomplished by adding a third camera to the configuration. With a proper geometrical arrangement of the cameras, the matching of a great majority of points can be accomplished quite efficiently, and the subsequent triangulation is strengthened in comparison with a two-ray triangulation. A fundamental property of such processes, whether with two or more than two cameras, is that the projector plays a passive role. It supplies the targets but does not otherwise participate in their determination.
The triangulation of targets is possible by virtue of the cameras"" having a known mutual position and orientation, collectively called xe2x80x9cpose.xe2x80x9d Each camera can acquire its pose by imaging known points in a given coordinate system, whether a system of the facility (global) or an arbitrary system established by a datum-generating device (local). Such known points are often called xe2x80x9creference points.xe2x80x9d If the projector should become a part of the position-determining system, its pose would likewise have to be known.
In several applications over the past decade, this approach has been addressed in the technical literature as well as in practice by rigidly mounting the cameras and the projector onto a bar or a frame. The mutual poses of the cameras and the projector are determined in a calibration procedure, which includes the calibration of the lens systems. The known positions of the dots on the projector slide then play a role similar to that of measured targets on the film or digital-image sensor, in that they can participate in the triangulation of the targets.
If the projector plays an active role, it can participate in the determination of the targets"" positions together with only one camera. Again, the mutual pose is calibrated, as are the lens systems. However, if a great many points are projected, the inherent ambiguities make such an application difficult. This technique has been mentioned mainly for the sake of comparison with other methods in which a projector plays an active role, such as Moirxc3xa9 techniques. The latter do not treat points, but lines. A light source projects a grid of straight lines onto a surface, where they are viewed through a similar grid by a camera or another receptor. The meshing of the sets of grid lines, deformed according to the object""s topography, makes it possible to map the surface. The key consideration here is that in these techniques the relative poses of the two optical systems must be known. As is explained in Non-Topographic Photogrammetry (ASRPS, 1989), in the section on xe2x80x9cSurvey of Instrument Types,xe2x80x9d in Appendix B, the components are rigidly coupled (aligned) to preserve the critical geometries. Examples feature integrated frameworks mounted on a base, etc.
Known systems in which a projector has an active role have been shown to have two characteristics:
1. The mutual configuration of the camera(s) and the projector must be known. Therefore, the systems are integrated into a rigid framework, and their poses are calibrated.
2. The optical properties of the lenses must be known in view of image compensation for distortion. Therefore, all optical systems are likewise calibrated.
There are several disadvantages to these properties and procedures. One of them is the complexity of rigidly integrated systems, which makes them cumbersome in industrial and/or hazardous environments, and which also makes them expensive. Such systems are not well adapted to the wide variety of measurements and measurement conditions often encountered in industry. If the situation calls for a different arrangement of the projector and the cameras than is possible with a given rigid system, the measurement cannot be performed without a rearrangement of this system on a new rigid mount, and recalibrating the entire configuration.
Another disadvantage is technical, in the sense that the systems must be recalibrated from time to time, as well as whenever even a minute physical alteration is suspected. This includes temperature changes, among others. The recalibration concerns the mutual configuration (item 1), the optical properties (item 2), or both.
Based on the preceding discussion, a need exists for a fundamental conceptual innovation, whereby a projector plays an active role in three-dimensional positional determinations, yet neither it nor the cameras are physically connected in any manner, and no extensive prior calibration of the projector or the cameras would take place. The cameras"" optical systems would have their parameters reasonably well known from past calibrations, while only a few parameters of the projector""s optical system would need to be known to some approximation, easily achievable without any actual measurement or testing. All the accurate calibrations would be performed simultaneously during the measurement process itself.
It is therefore an object of the present invention to provide a system and method for measuring geometric features of an object.
It is an additional object to provide such a system and method that eliminate ambiguities inherent in two-ray triangulation.
It is a further object to provide such a system and method that use a projector in three-dimensional positional determination.
It is another object to provide such a system and method that do not require an extensive calibration.
These objects and others are attained by the present invention, the method aspect of which is for characterizing at least a portion of a surface of an object. The method comprises the steps of projecting from a projector having no predetermined pose a plurality of images onto a surface desired to be characterized. The images comprise a first set of images and a second set of images distinguishable from the first set. All the first and the second set of images have known two-dimensional coordinates relative to the projector.
A pose of a first and a second camera positioned in spaced relation from each other is then determined, as are the three-dimensional coordinates of at least some of the first set of images on the surface, with the use of the first and the second cameras. From the three-dimensional coordinates the pose of the projector is determined.
Next the three-dimensional coordinates of at least some of the second set of images are determined using knowledge of the projector pose and the first and the second camera. From the three-dimensional coordinates of the first and the second set of images, then, the surface can be characterized. Thus the projector can be used as a third camera without the need to rigidly integrate and calibrate the whole system.
The system of the present invention comprises a projector, a slide having a set of opaque elements thereon to form the first and the second set of images on the surface to be characterized, at least two cameras, and an analysis routine resident in a processor.
The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to be expressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing.